Maiken H. Mikkelsen

Point‑of‑care testing (POCT) demands diagnostic platforms that pair high analytical sensitivity with compact, low‑cost readout hardware, yet fluorescence‑based immunoassays face a fundamental tradeoff between detector cost and detection performance. Plasmonic nanocavities—such as metal–dielectric–metal nanopatch antennas—offer a powerful signal‑amplification strategy by dramatically enhancing fluorophore emission, enabling sensitive fluorescence detection with inexpensive optics. Here, we introduce a plasmonically enhanced fluorescence microarray immunoassay that integrates nanopatch antennas for high‑sensitivity, low‑cost quantification of key heart‑failure biomarkers, NT‑proBNP, NGAL, and galectin‑3, using a custom smartphone‑based detector. The nanopatch antennas yield up to 485‑fold fluorescence enhancement enabling quantitative and multiplexed detection with limits of detection of 0.055, 3.932, and 0.864 ng/mL, respectively. Analysis of NT‑proBNP–positive human samples shows strong correlation with clinically validated assays, demonstrating the potential of this plasmonically enhanced platform to deliver high‑performance heart‑failure biomarker monitoring at low cost in resource‑limited and point‑of‑care settings.

Plasmonic nanogap cavities enable extreme confinement of optical fields, and consequently enhanced light-matter interactions. However, light-matter coupling of molecules within these structures remains difficult to control, because the coupling strength is sensitive to the orientation of the molecular transition dipole moment relative to the cavity field. Here, we demonstrate a molecular design strategy for engineering preferential dipole alignment in nanoparticle-on-mirror plasmonic nanogap cavities. A series of ruthenium(II) polypyridyl emitters, bearing diazonium-forming surface anchoring groups at different positions on the ligands, were electrochemically grafted onto aluminum oxide-coated gold surfaces and integrated into plasmonic nanogap cavities. The binding group placement was designed to impose distinct transition dipole angles relative to the surface and, consequently, relative to the dominant cavity electric field. Photoluminescence and time-correlated single-photon counting measurements showed that cavity integration produces strong emission enhancement and large Purcell factors, with maximum Purcell enhancement increasing systematically as the estimated transition dipole moment becomes more closely aligned with the cavity field. Finite-element electromagnetic simulations reproduced the observed angular trend and demonstrated that spatial heterogeneity in the nanogap field gave rise to broad distributions of excited-state decay rates. These results establish molecular design as a practical strategy for engineering light–matter interactions and identify surface attachment chemistry as a scalable route to tuning light–matter coupling in plasmonic nanogap cavities.

Thermal photodetectors, including thermoelectric and pyroelectric detectors, are critical for the detection of long‐wave infrared light, but they are limited by slow response times due to thermal diffusion. If thermal photodetectors with high speeds and sensitivities can be realized, they would allow for flexible sensing at any wavelength, as all selectivity of the absorber would be reproduced in the detectivity. Here, a room‐temperature, pyroelectric photodetector is reported that couples a thermally‐sensitive aluminum nitride (AlN) layer with a metallic metasurface that acts as an efficient, spectrally‐selective photothermal converter. These pyroelectric detectors exhibit record‐breaking speeds, with 3 dB bandwidths up to 2.8 GHz, which corresponds to a rise time of 125 ps. These ultrafast speeds are attained while maintaining competitive responsivities and noise equivalent powers as small as 96 pW √Hz ⁻¹ . By varying the active area of devices, it is determined that even the smallest attainable devices are limited by their resistance‐capacitance (RC) time constants, and finite element simulations suggest that thermal response times as fast as 30 ps may be realized. These metasurface‐enabled pyroelectric photodetectors approach the speeds of commercial semiconductor photodiodes, contrary to the common conception of thermal detectors as slow, and present opportunities for advanced sensing such as spectrally broad multispectral imaging or polarimetry.

Thin‐film interference is widely leveraged in classical optics as a minimalistic yet powerful lever for controlling optical fields, underpinning technologies from anti‑reflective coatings to photovoltaics. However, extending this concept to diamond membranes and other thin film solid‐state quantum emitter host materials has remained unexplored, partly due to the challenges in fabricating ultrathin membranes. Here, a wedge‐shaped diamond membrane is engineered to demonstrate thickness‐dependent interference phenomena. This can not only be used as a broadband reflector or wavelength‐specific absorber, but it can also significantly modulate both excitation and emission intensities of silicon vacancy (SiV) centers, leading to a photoluminescence enhancement up to 96‐fold. This route circumvents the need for high‑Q cavities or intricate nanolithography, and the sample gradient offers a visualization of broadband resonances and anti‑resonances across the visible to near‑infrared spectrum. These findings offer an alternative method for controlling effective brightness enhancement or suppression of diamond color centers‐based quantum photonics. Beyond quantum emission enhancement, diamond membranes could serve as a passive layer such as a broadband reflector or a wavelength‐specific absorber, and find applications in heterostructures, photovoltaics, display technologies, and even in semiconductor thermal management.

Nanogap cavity metasurfaces – an array of metallic nanoparticles separated from a metal plane by a nanometer‐scale dielectric material – can manipulate electromagnetic waves across a wide wavelength range. Through this, they can profoundly modify the optical processes of molecules and materials relevant to quantum communications, photocatalysis, and optoelectronics. Interactions between nanocavities and light, however, require overlap between the cavity resonance and the energy of the incident photon or optical transition, demanding labor‐intensive fabrication of bespoke metasurfaces for each desired application. Here, we dynamically tune the resonance wavelength of nanogap cavity metasurfaces by modulating the refractive index of the surrounding medium using solvents. We achieve precise, reversible, and broadband resonance control for narrow nanogap cavity resonances (full width half max n = 1.3–1.7 without any discernable metasurface degradation. This opens new possibilities for applications in optical sensing with significantly increased nanofabrication tolerances, such as tunable photonic devices and adaptive optical systems, where precise control over light–matter interactions is critical.

Artificially tailoring quantum emitters by coupling them to optical cavities is critical for applications such as all-photonic quantum computing and quantum key distribution. Among quantum emitters, diamond defect centers, with their atomic-like characteristics, have drawn intense interest for quantum applications. However, the spontaneous emission lifetime of single photons from diamond defects has been limited to several hundred picoseconds, partly due to challenges in realizing cavities in diamond with sufficiently small mode volumes. Here, we sandwich an only 10 nm diamond membrane with implanted negatively charged silicon vacancy centers (SiV^-) between a gold mirror and arrays of nanodisks, resulting in ultrafast single-photon sources with lifetimes as short as 5.5 ps. This corresponds to an improvement in the radiative spontaneous emission rate of over 700-fold, also termed the Purcell factor, and is the result of a greatly enhanced local density of states in the ultrasmall mode-volume cavities. Furthermore, the cavity-coupled single SiV^- centers exhibit a 4800-fold photoluminescence enhancement with a single-photon generation rate of up to 361 Mcps at room temperature. These results could refine the engineering of diamond defect centers and position other solid-state platforms as strong quantum-information contenders when coupled to ultrasmall mode-volume cavities, thus illustrating the potential for large-scale, artificially tailored single-photon sources.

Single photons are cornerstones of quantum technologies. The need for an ultrafast, bright, and stable photon source emitting in the telecom-band at ∼1550 nm, ideally operating at room temperature, has resulted in a decades-long quest. However, to date, telecom sources are hampered by inherently long radiative lifetimes that severely limit brightness and speed, material instability, or the need for cryogenic operation. Here, stable colloidal PbS/CdS quantum dots emitting at 1550 nm (C-band) or 1350 nm (O-band) are embedded in a solution-synthesized nanoparticle-on-mirror cavity. Single cavity-coupled quantum dots experience extreme Purcell factors up to 10,700, resulting in ultrafast emission lifetimes of 65 ps, along with near-complete blinking suppression. As a result, 12 million single photons are emitted per second affording a single photon source at 1550 nm that is more than two orders of magnitude brighter than previously possible at room-temperature. These telecom-band single photon sources are solution-processable and lithography-free and may leverage mature colloidal fabrication technologies for future quantum applications.

Silicon vacancy centers (SiVs) in diamond have emerged as a promising platform for quantum sciences due to their excellent photostability, minimal spectral diffusion, and substantial zero-phonon line emission. However, enhancing their slow nanosecond excited-state lifetime by coupling to optical cavities remains an outstanding challenge, as current demonstrations are limited to ∼10-fold. Here, we couple negatively charged SiVs to sub-diffraction-limited plasmonic cavities and achieve an instrument-limited ≤8 ps lifetime, corresponding to a 135-fold spontaneous emission rate enhancement and a 19-fold photoluminescence enhancement. Nanoparticles are printed on ultrathin diamond membranes on gold films which create arrays of plasmonic nanogap cavities with ultrasmall volumes. SiVs implanted at 5 and 10 nm depths are examined to elucidate surface effects on their lifetime and brightness. The interplay between cavity, implantation depth, and ultrathin diamond membranes provides insights into generating ultrafast, bright SiV emission for next-generation diamond devices.

The nonlinear conversion of photons from lower to higher energy is important for a wide range of applications, from quantum communications and optoelectronics to solar energy conversion and medicine. Triplet-triplet annihilation upconversion (TTA UC), which utilizes an absorber/emitter molecular pair, is a promising tool for upconversion applications requiring low intensity light such as photovoltaics, photocatalysis, and bioimaging. Despite demonstrations of efficient TTA UC in solution, practical applications have proven difficult, as thin films retard the necessary energy transfer steps and result in low emission yields. In this work, TTA UC emission from a thin film is greatly enhanced through integration into plasmonic nanogap cavities consisting of a silver mirror, a nanometer-scale polymer spacer containing a TTA molecular pair, and colloidally synthesized silver nanocubes. Mechanistic studies performed by varying the nanocube side length (45-150 nm) to tune the nanogap cavity resonance paired with simulations reveal absorption rate enhancement to be the primary operative mechanism in overall TTA UC emission enhancement. This absorption enhancement decreases the TTA UC threshold intensity by an order of magnitude and allows TTA UC emission to be excited with light up to 120 nm redder than the usable wavelength range for the control samples. Further, combined nanogap cavities composed of two distinct nanocube sizes result in surfaces which simultaneously enhance the absorption rate and emission rate. These dual-size nanogap cavities result in 45-fold TTA UC emission enhancement. In total, these studies present TTA UC emission enhancement, illustrate how the usable portion of the spectrum can be expanded for a given sensitizer-emitter pair, and develop both mechanistic understanding and design rules for TTA UC emission enhancement by plasmonic nanostructures.

Precise charge dynamics could help to improve the operation of solar cells and sensors.